Everybody knows strep throat is as common as backpacks among school children, but one of the surprises in a new study of Pittsburgh students is that it isn't even more common.

Elementary students "are sitting in a sea of strep," said Dr. Judith Martin, an infectious disease specialist at Children's Hospital who led a four-year study of the prevalence of group A Streptococcus bacteria in a Pittsburgh private school.

The researchers report in this month's issue of the journal Pediatrics that six out of every 10 students tested positive for strep at least once during a school year.

But just because 60 percent tested positive to strep at one time or another doesn't mean that all of them got sick. In fact, a bit more than half of those who tested positive had no symptoms.

It is this latter finding, regarding a group of children called "carriers," that is the most interesting. A child who is a carrier might have the bacteria present for a few months or even a few years and, once a carrier, is likely to become a carrier again and again.

What's more, a carrier doesn't necessarily have just one genetic type of strep A bacteria. Rather, he might harbor a succession of different types of strep bugs.

"We've known for a long time that there are kids who are prone to being carriers," said Dr. Stanford Shulman, chief of infectious diseases at Children's Memorial Hospital in Chicago. But the findings by Martin and her Pittsburgh colleagues are of particular interest because it might help researchers finally figure out what makes those children prone to being carriers.

And just as researchers don't understand what causes some children to be carriers, they don't know why some appear to be largely immune from the bacteria.

In the short term, the finding has led Children's Hospital of Pittsburgh to change the way it treats carriers who report classic strep throat symptoms and to recommend that other doctors in the Pittsburgh area follow suit.

Strep throat usually can be treated effectively with antibiotics, but diagnosing the disease remains a bit of a guessing game, even with the advent of rapid strep tests. If a patient has classic symptoms ---- a sore, red throat with white patches on the tonsils ---- and tests positive for strep, a doctor will assume the patient has strep throat and prescribe antibiotics.

But just because a patient has strep throat symptoms and a positive test doesn't mean he has strep throat, Martin said. In some cases, the patient is simply a strep carrier with symptoms caused by a virus. In those cases, antibiotics don't cure the infection.

Carriers often are identified when they have a succession of sore throats that resemble strep throat but don't respond to antibiotics, Martin said. Antibiotics can't rid a carrier of the strep bacteria ---- the bugs in carriers are in a dormant state and less susceptible to antibiotics ---- so doctors often stop prescribing antibiotics to these patients, even when they have classic strep throat symptoms.

But Martin said the new study raises questions about whether that is wise. Because carriers don't have just one type of bacteria, but tend to harbor a succession of different strains of group A strep, it's possible that one of those strains might escape dormancy and become an active infection.

Failure to diagnose strep throat in a carrier could allow the infection to progress to rheumatic fever, which can cause arthritis and heart valve damage.

Antibiotics have made rheumatic fever rare in most parts of the United States, but Pittsburgh, like Salt Lake City, continues to see some cases; Children's handles about a dozen each year.

"When we have new physicians who come to the area, most of them have never seen rheumatic fever before," Martin said.

Because of that continued prevalence of rheumatic fever in the region, Children's has decided to start treating carriers who have strep throat symptoms with antibiotics, reducing the chance that a real strep throat case will go untreated.

Shulman, in Chicago, said the idea that carriers are more at risk for developing rheumatic fever is still speculative. Martin acknowledged that the study saw no evidence of carriers developing this more serious strep A infection.

Many carriers likely go undetected; after all, they don't develop strep throat symptoms. In the four-year study, however, researchers swabbed the throats of students at Oakland's Falk School every two weeks throughout the school year. A total of 125 children participated in the study ---- 70 percent of them for the full four years.

In addition to Martin, the research team included Karen Barbadora, Dr. Michael Green and Dr. Ellen Wald, the chief of allergy, asthma and infectious diseases at Children's.

Earlier findings from the same study had helped identify an unusually high rate of strep bacteria that were resistant to common antibiotics, such as erythromycin. Though later studies confirmed that this 10 percent rate of resistance was found throughout the Pittsburgh area, Shulman said that a national strep A surveillance study that he directs never found similar rates of the resistant strains.